(1) Field of the Invention
The present invention relates to a private automatic exchange (PBX) system for controlling a calling party address in an Integrated Systems Digital Network (ISDN public network), and more particularly, to a connecting system between a terminal connected to an ISDN public network and an extension terminal accommodated by a PBX system.
To connect an ISDN public network and a PBX system, it is required to be able to deal with an extension terminal accommodated by the PBX system on an equal basis with a terminal connected to the ISDN.
(2) Description of the Related Art
In a conventional PBX system, as described later in more detail with reference to the drawings, when a call is effected from a terminal connected to the ISDN public network to an extension terminal accommodated by the PBX system, the extension terminal receives the calling party address from the calling party terminal through the ISDN public network and a PBX network in the PBX system. The called party extension terminal, however, could not identify whether the received calling party address is from a terminal connected to the ISDN network or from another extension terminal accommodated by the PBX system. This was inconvenient for the user of the PBX system. Namely, for example, if the user is away from the called party extension terminal, when a call is made to the extension terminal. After the termination of the call, if the user wants to return the call to the person at the calling party terminal, the user cannot call back to the calling party terminal because the user has no way of knowing whether the calling party was from the ISDN network or from another extension terminal accommodated by the PBX system.